troubleshooting the thermostat/relay/circpump triad

I have a boiler unit in my house that heats a 3 zone baseboard heating system.

One of the zones is only getting intermittent heat and I'm having trouble
sorting out whether the culprit is the thermostat, relay or circ pump.

I checked the other zones' thermostats and it seems that the "command" for heat on is
to short the leads, so I disconnected one lead from the thermostat and shorted
the low voltage leads on the relay box. This yielded the satisfying clunk of
the solenoid engaging, the slight hum of the suspect zone's circ pump and the
zone's feed pipe getting hot.

So, I thought "done" - the culprit was the thermostat.

However, I left the leads shorted for about 1 hour while I cleaned up all the
periphery and tended to other things. I came back to find the feed pipe to
that zone again cold. The boiler was running and the solenoid still responded
to short the low voltage leads, but it was clear (at least to me) that the
circ pump wasn't moving water.

So, I see these possibilities to explain the intermittent behavior.

1) the solenoid is physically engaging but the relay isn't reliably
providing power to the circ pump,
or
2) the circ pump is near dead and intermittent.

This is an old system, and I don't know what type of voltage is going through
the relays to the circ pumps.

The thermostat probably doesn't directly turn on the pump and the zone valve, it could the zone valve but if it went directly to the pump then it would be cross connected to the output of the other thermostats and therefore turn on the other zone valves, and to be honest with you I don't remember if the zone valves I use have a separate output to turn on the pump. So look for a relay controlled by the thermostat that controls the pump, or a separate output from the zone valve that would control the pump.

OK to follow up, I went and looked at the instructions for my TACO zone valves and they have 3 terminals, one of them goes to your boiler control box to hook to one of the "T" connections, to turn on the relay to the circulation pump. So it could be a problem with your zone valve...

I seem to have a similar problem with my oil boiler with white control. The relay rattles but does not stay engaged and therefore the circulator does not stay on. need to know what provides the voltage to the solenoid? There is also a small transformer in the unit.

Is this an ice cube relay? If it is then change it on general principles.

A little more seriously, the circulators should be 110v and just about everything else should be 24 vac.

It sounded like everything was starting up ok (you said you could feel heat in the supply side, could you feel heat in the radiators?). If it's shuting down mid-cycle you'll have to wait for that to happen and then see where either the power stops or a valve closes (if it has zone valves) or a circulator shuts down. If they are Taco circulators the power ends can be swapped with the power end from a good zone to see if that eliminates the problem. Cannibalizing is sort of a last resort but it is an option.